1. Field of the Invention
This invention relates to methods and apparatuses for directionally solidifying molten metals, most particularly to the production of single crystals with controlled crystallographic orientation.
2. Description of the Prior Art
lt is well known that great improvements in the performance of metal structures can be achieved by unidirectional casting techniques which produce articles with columnar grain or single crystals. See, for example, the teachings of Ver Snyder, U.S. Pat. No. 3,260,505 and Piearcey, U.S. Pat. No. 3,494,709. The principal objective of the prior art apparatus, methods, and articles has been to provide structures which have enhanced properties along the principal axis of the article, that is, the principal axis of the article is typically the solidification growth axis or the axis along which the solidification front is caused to move.
When metals are directionally solidified, they often by nature solidify or grow faster in one crystallographic orientation than others. For example, in nickel base superalloys the &lt;001&gt; orientation is found to predominate. As a result, single crystal castings made by means disclosed in U.S. Pat. No. 3,494,709, mentioned above, will have the &lt;001&gt; orientation lying along the growth axis. Therefore, to produce another crystallographic orientation along the principal axis of solidification specialized techniques must be used. Also, the orientation of crystals with respect to the plane perpendicular to the axis of solidification is random in most directionally solidified articles unless steps are taken to achieve control. The crystallographic orientation measured along the principal axis of a casting is called the primary orientation, while the polar orientation in the plane perpendicular to the principal axis is called the secondary orientation.
The properties of a material such as a columnar grain or single crystal material are influenced by its crystallographic orientation. For example, the elastic moduli will be importantly varied in many alloys and the performance of parts under stress and strain thereby affected. Consequently in more sophisticated applications of advanced materials, it is of increasing importance to control both the primary and secondary orientations. The crystallographic orientations of materials are determinable by conventional nondestructive laboratory techniques. Radiographic diffraction, e.g. by the Laue method, is most useful. Furthermore, changes in crystallographic structure can be readily ascertained by conventional grain etching. If the orientation at a location in a part is determined, the orientation will be the same in another region in the absence of an intervening grain boundary, and absent subtle crystal variations beyond the scope of this discussion.
A useful technique for controlling crystallographic structure in cast articles is the use of a previously made metal seed which has the desired structure. If the article casting can be made to grow epitaxially from the seed, the seed structure will be reproduced.
Of course, growing objects from seeds is well known. For instance the Bridgman method for epitaxial single crystal formation disclosed in U.S. Pat. No. 1,793,672 and other publications dates from the 1920's. Delano in U.S. Pat. No. 2,791,813 describes structures with controlled crystallographic orientations in which seed crystals are used to attain the desired result. Barrow et al in U.S. Pat. No. 3,759,310 describes an apparatus and electric arc method for making single crystal articles with a consumable electrode in which a seed crystal at the bottom of the mold is used. More recently, Petrov et al in U.S. Pat. No. 3,857,436 describes an improved method and apparatus for manufacturing single crystal articles. Disclosed therein are means and methods for initiating crystallization at a conical-shaped bottom chamber where abrupt super-cooling conditions are created. Petrov U.S. Patent describes further refinements. Also, Copley U.S. Pat. No. 3,598,169, discloses the casting of relatively flat objects using seed wedges and accomplishing radially outward solidification.
With the exception of Barrow, all the aforementioned techniques anticipate heating the mold prior to the introduction of the molten metal. The practice in the prior art is that the seed is in the mold during the heating. Therefore, it is also heated with the mold to a relatively high temperature although in some situations its location would indicate lesser heating. As the superheated molten metal is introduced into the mold and allowed to stabilize, it contacts the heated seed and causes it to partially melt. Of course it is necessary to melt at least part, but only a part, of the seed, and this necessitates a control over the initial and transient conditions of the seed, mold, molten metal, and other influential factors.
Much of the prior art reflects laboratory technique and is not oriented toward mass production. Now, there is a trend towards greater commercial utilization of articles having controlled crystallographic structure, such as columnar grain and single crystal gas turbine airfoils. This has impelled the development of automated casting techniques to produce articles in quantity on an economic basis. According to one of these techniques, described in King et al, U.S. Pat. No. 3,895,672, a heated mold is clamped onto a cool chill plate just immediately prior to the introduction of molten metal into the mold. If the seed crystal is used, it is attached to the chill plate and it is, of course, correspondingly cool. The short duration between the mating of the hot mold and the cool chill plate provide little time for the temperature of the seed to increase. The same difficulty can obtain in some of the prior art apparatus and methods. If the seed is too cold, insufficient melting will occur and epitaxy will not result. One method of overcoming this is to increasingly superheat the molten metal but to do so is disadvantageous since superheating often leads to increased time and cost, undesired vaporization of elements, and increased degradation of the mold. To separately heat the seed or to include the seed with the mold when the mold is being heated after the methods of the older art is also disadvantageous, both from the mechanical and manufacturing complications and because the seed can become unduly oxidized or otherwise contaminated.
Another consideration during the manufacture of articles of controlled primary and secondary crystallographic orientation is that after manufacture, the orientation of the seed must be, first, accurately defined by suitable inspection techniques and, second, controlled precisely with respect to the axes of the articles being cast. Accordingly, the providing of seeds for casting can represent a significant cost. It is, therefore, desirable that seeds be recovered from the casting process after the article is formed and reused if possible. However, when the seed is severely melted during the casting operation or surrounded by a larger quantity of solidified metal of extraneous orientation, recover for reuse is difficult.